The present invention relates to a composite target material used in the sputter-production of a thin film made of a rare earth metal and a transition metal (iron-group metal) that is attracting the increasing attention of researchers because of its potential for use as a magnetooptical recording material. The invention also relates to a process for producing such composite target material.
There exist three types of target material that are currently used in the sputter-production of thin films from combinations of rare earth metals and transition metals (iron-group metals). The first is an alloy target material with an oxygen content of 0.5-3.0 wt % that is produced by melting two dissimilar metals in an electric arc furnace either in vacuo or in an inert gas atmosphere (this type of composite target material consists of 30-50 wt % of a rare earth metal and 70-50 wt % of a transition metal). The second type is also an alloy target material but it is a composite comprising rare earth metal chips placed on a transition metal sheet or the combination of transition metal chips placed on a rare earth metal sheet. The third type is an alloy target material produced by a process patent previously applied for by the present inventors, this process consisting of preparing an alloy ingot by arc melting, dividing the ingot into fine particles and hot-pressing the resulting alloy powder into a compact.
The process for producing the first type of alloy target material has the following disadvantages:
(a) It is liable to segregation of trace elements during arc melting; the alloy product has residual cavities and casting defects; because of the brittleness of the alloy, it defies the use of forging, heat treatment and other processes conventionally used to provide homogeneity in the structure and composition of alloys, and this causes considerable difficulty in obtaining an ingot of a homogeneous composition, so that the resulting target material, and hence the film formed by sputtering said target, is liable to have an uneven composition;
(b) The size of the target material obtained depends on the size of the electric arc furnace used, and the brittleness of the material presents considerable difficulty in producing a product having an increased cross-sectional area; and
(c) Arc melting alone is not sufficient to produce alloy target materials of desired shapes (especially thin-walled materials), and in order to obtain the desired shapes, subsequent working such as cutting or grinding is necessary, but then this greatly reduces the process efficiency.
The first type of alloy target material produced by the method described above has the following defects:
(d) Because of its low tenacity (deflective strength .ltoreq.2 kg/mm.sup.2), the target material will easily crack and is difficult to handle;
(e) It is highly likely to crack by thermal shock applied during sputtering procedures;
(f) The target material has a high oxygen content (0.5-3.0 wt %), so the thin film produced by sputtering this target material is not highly adapted to the formation of a perpendicular magnetization film for use in magnetooptical recording;
(g) The size of the target material depends on the size of the electric arc furnace used, and the maximum diameter that can be obtained with the state of the art today is 60 mm; and
(h) The deposition rate obtained by sputtering this target material in a magnetron (Ar partial pressure: 1.times.10.sup.-2 Torr, output: 0.5 A.times.145 V, pre-sputtering time: 30 min., distance between slide glass substrate and target: 70 mm, bias voltage: 0 V, rotational speed of substrate: 10 rpm) is relatively slow (1,000-2,000 .ANG./min).
The process for producing the second type of composite target material has the following disadvantages:
(a) Abnormal discharge will easily take place between the sheet and chips; and
(b) Non-uniformity in the distribution of rare earth metal chips and trace alloying elements on the transition metal sheet renders it difficult to obtain a film with a uniform composition.
The target material produced by this method has the following defects:
(c) It cannot be rotated or inverted and great difficulty is involved in using it while ensuring uniform distribution of the chips;
(d) Magnetic fluxes have a tendency to be concentrated in the bulk of the sheet rather than on its surface, and the chips on the sheet prevent the formation of a uniform magnetic field; and
(e) As in the case of the first type of composite target material, the deposition rate obtained by magnetron sputtering (assuming sputtering conditions which are the same as those described in (h) above) is relatively slow (1,000-2,000 .ANG./min).
The third type of alloy target material and the process for producing it have the following defects:
(a) The target material is made of a powder of an intermetallic compound, and hence, is brittle; and
(b) An oxide will easily form on the powder surface so as give a product having a higher oxygen content than the first and second types of target material, and therefore, if it is handled improperly during sputtering procedures, it will produce a thin film that is not suitable for use in a perpendicular magnetic recording.